Buckwheat is thought of as a grain but it's really more closely related to sorrel and rhubarb. Before the use of modern nitrogen fertilizers, a post WW2 use of excess bomb materials, buckwheat was a staple food. The true grains we use nowadays require much more nitrogen.Buckwheat has a quick life cycle and thrives in acid soil with low nitrogen. This makes it great for preppers/homesteaders for several reasons. It can produce a pseudo grain crop that has 18% protein in about 90 days. That's food for the family or livestock. If tilled in just after blooming in about 30 days, it provides bees with nectar for a nice dark honey and can then be tilled in as a green manure. It also suppresses weeds so it makes a good cover crop.

It can be boiled and eaten or ground into flour for most things you would use wheat flour to make. Pancakes, bread, pasta etc. It can even be fermented for beer and whiskey. It is gluten free so it's great for those with celiac disease.
The hulls can be used as filler for pillows or bedding. The leaves can be used in tea and even has positive effects in those with diabetes.
Don't leave buckwheat out of your plans. Waiting for wheat, barley and oats to mature may not be an option. A crop of buckwheat can hold you off until those crops can be harvested. I grow it for the bees and let the chickens forage through and get the seed.
On the downside, it can run rampant through a pasture if allowed to go to seed but that's an easy fix. Buckwheat can also be planted in the pasture as forage or cut for hay at 1-3 tons per acre.